The higher aspect ratio of a vertical tail surface (higher at least when compared to the fuselage) will allow to make the tail smaller, but causes an earlier stall.
-Peter Kämpf said so here
How does the vertical tail, and more specifically, the aspect ratio of the vertical tail, impact stall?
Generating lift creates downwash. Downwash reduces the effective angle of attack:
A high aspect ratio reduces the downwash and thus also its reducing effect on the effective angle of attack.
Therefore a high aspect ratio wing will experience a higher $\alpha_{eff}$ than a low aspect ratio wing at the same angle, and thus stall earlier.
The same happens with the rudder, but then with $\beta$ instead of $\alpha$.
EDIT: clarifying alpha - beta similarity
So, in the case of sideslip, we have the following situation:
See the similarity between the $\alpha$-image above, and the $\beta$-image. The mass flow changes direction, and leaves the end of the rudder (more or less) parallel to the rudder chord line. This deviation in airflow direction causes the force to the left.
So for a normal wing ($\alpha$-image), flow enters straight, leaves going down (downwash) and as a consequence the wings give a force up.
For the rudder ($\beta$-image), the flow enters straight, leaves going to the right (downwash, but then sideways) and as a consequence the rudder give a force to the left.
About the stalling, stalling is not limited to the wings, also rudders can stall if the maximum effective angle they experience is above stalling angle ,and the force generated suddenly drops.
If we compare two rudders both with the same airfoil and the same $\beta_{eff_{stall}}$, say 10°. The $\beta_{stall}$ will be given by:
$$ \beta_{stall} - \beta_{downwash} = 10° $$
If the $\beta_{downwash}$ is 4° for the low AR wing, and 2° for the high AR wing, it will mean the low AR wing will stall at a $\beta$ of 14° and the wing with the high AR will have a $\beta_{stall}$ of 12°.
Peter Kämpf would be the one to answer this definitively, but in the context of his full answer it's clear that he is referring to the function of the vertical stabiliser during sideslip.
If an aircraft is slipping the flow will have some component perpendicular to the vertical tail fin, and thus the fin will have some angle of attack. Sideslip too steeply and the fin will stall, just like any other aerofoil.
and the fin will stall. Maybe this is what Peter was saying, and I read it as and the plane will stall. I may have read more into that statement than was there.
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